As the size of functional elements in integrated circuits decreases, complexity and interconnectivity increases. To accommodate the growing demand of interconnections in modem integrated circuits, on-chip interconnections have been developed, and such interconnections generally consist of multiple layers of metallic conductor lines embedded in a low dielectric constant material. The dielectric constant in such material has a relatively important influence on the performance of an integrated circuit. Materials having low dielectric constants (i.e., below 2.2) are desirable because they typically allow faster signal velocity and shorter cycle times. Moreover, lowering of the dielectric constant often reduces capacitive effects, leading frequently to reduced cross talk between conductor lines and lower voltages to drive integrated circuits.
One way of achieving low dielectric constants is to select materials with inherently low dielectric constants. Generally, two different classes of low dielectric constant materials have been employed in recent years--inorganic oxides and organic polymers. Inorganic oxides often have dielectric constants between 2.5 and 4, which tends to become problematic when device features are smaller than 1 .mu.m. Organic polymers, including epoxy networks, cyanate ester resins and polyimides vary greatly in their usefulness as low dielectric material. Epoxy networks frequently show disadvantageously high dielectric constants at about 3.8-4.5. Cyanate ester resins have relatively low dielectric constants between approximately 2.5-3.7, but tend to be rather brittle, thereby limiting their utility. Polyimides have shown many advantageous properties including high thermal stability, ease of processing, low stress/thermal coefficient of expansion (TCE), low dielectric constant and high resistance. Polyimides are therefore frequently used as alternative low dielectric constant polymers.
With respect to other properties, desirable dielectrics should also be free from moisture and out-gassing problems, have suitable adhesive and gap-filling qualities, and have suitable dimensional stability towards thermal cycling, etching, and CMP processes (i.e., chemical mechanical polishing). Suitable dielectrics should also have Tg values (glass transition temperatures) of at least 300.degree. C. and preferably 500.degree. C. or more.
The demand for materials having a dielectric constant of lower than 2.2 has led to the development of dielectric materials with designed-in nanoporosity. Since air has a dielectric constant of about 1.0, it has become a major goal to reduce the dielectric constant of nanoporous materials down towards a theoretical limit of 1.
Several approaches are known in the art for fabricating nanoporous materials. In one approach, a thermostable polymer is blended with a thermolabile (thermally decomposable) polymer. The blended mixture is then crosslinked and the thermolabile portion thermolyzed, and examples for this approach are set forth in U.S. Pat. No. 5,776,990 to Hedrick et al. In another approach, thermolabile blocks and thermostable blocks alternate in a single block copolymer. The block copolymer is then heated to thermolyze the thermolabile blocks. In a third approach, thermostable blocks and thermostable blocks carrying thermolabile portions are mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. In yet a fourth approach, small hollow glass spheres are introduced into a material. Examples are given in U.S. Pat. No. 5,458,709 to Kamezaki and U.S. Pat. No. 5,593,526 to Yokouchi.
Regardless of the approach used to introduce the voids, structural problems are frequently encountered in fabricating nanoporous materials. Among other things, increasing the porosity beyond a critical extent (generally about 30% in the known nanoporous materials) tends to cause the porous materials to collapse. Collapse can be prevented to some degree by adding crosslinking additives that couple thermostable portions with other thermostable portions, thereby producing a more rigid network. U.S. Pat. No. 5,710,187 to Streckle, Jr., describes crosslinking for this purpose, crosslinking aromatic monomers using multifunctional acyl- or benzylic halides.
To achieve such a high degree of crosslinking, two different strategies are typically employed. In one strategy, a very high density of crosslinking functionalities is incorporated into the polymeric strands. A high density, however, may create several problems. First, more synthetic steps are usually needed to generate appropriate monomers or blockpolymers. Second, a very high density of crosslinking functionalities may unfavorably alter the chemical or physicochemical nature of the desired end product. Third, the balance of crosslinking between polymeric strands and crosslinking within a single polymeric strand is generally difficult to control. Moreover, in the process of crosslinking, the flexibility of the polymeric strands decreases. Decreased flexibility of polymeric strands may tend to makes subsequent reactions of unreacted crosslinking functionalities less likely, leading to an undesirably high number of excess reactive groups. Such excess reactive groups may then interfere with down-stream reactions. Another disadvantage of decreased flexibility of polymeric strands is that an even distribution of thermolabile portions may be constrained.
In the other strategy, a relatively large amount of exogenous crosslinking molecules is added to promote a high degree of crosslinking. Such amounts may create additional problems. One problem is that exogenous crosslinking molecules must generally be chemically compatible with the polymeric strands, i.e., they need to be soluble in the same solvent system, and they need to exhibit reactivities specific towards polymeric strands. The addition of exogenous crosslinking molecules generally does not afford control over crosslinking between polymeric strands and crosslinking within a single polymeric strand. Furthermore, in the process of crosslinking, the flexibility of the polymeric strands decreases. Decreased flexibility of polymeric strands, in turn, promotes steric hindrance and makes subsequent crosslinking reactions less likely. Therefore, the addition of exogenous crosslinking molecules has inherently limited efficiency. Still further, unreacted excess crosslinking molecules may not be easily removable from the crosslinked material, and may adversely influence the physicochemical properties of the end product, or may interfere with downstream reactions.
Even if crosslinking is successfully performed to a high degree, many crosslinkers known in the art suffer from a common further disadvantage. In general, crosslinkers comprise at least one sigma bond, allowing an extra degree of flexibility--typically rotational freedom along the bond axis. This contributes, despite a covalent connection, to a somewhat reduced degree of rigidity.
In summary, various methods are known to crosslink polymers in nanoporous materials. However, current methods often tend to have inherently limited efficiency, or still allow undesirable flexibility in the crosslinked polymeric strands. Surprisingly, despite great efforts to improve crosslinking in nanoporous materials, there is no method that permits crosslinking with very high efficiency, without adding at least some degree of flexibility in the crosslinked polymeric strands. Therefore, there is still a need for methods and compositions that circumvent these limitations.